In October 1988, Bill Denbrough gives his seven-year-old brother, Georgie, a paper sailboat. Georgie sails the boat along the rainy streets of small town Derry, and is disappointed when it falls down a storm drain. As he attempts to retrieve it, Georgie sees a clown in the sewer, who introduces himself as Pennywise the Dancing Clown. The clown entices Georgie to come closer, then severs his arm and drags him into the sewer.
The following summer, Bill and his friends—Richie Tozier, Eddie Kaspbrak, and Stan Uris—run afoul of older bully Henry Bowers and his gang. Bill, still haunted by Georgie’s disappearance and the resulting neglect from his grief-stricken parents, discovers that his brother’s body may have washed up in a marshy wasteland called the Barrens. He recruits his friends to investigate, believing his brother may still be alive. Ben Hanscom learns that the town has been plagued by unexplained tragedies and child disappearances for centuries. He is targeted by Bowers’ gang, after which he flees into the Barrens and meets Bill’s group. They find the sneaker of a missing girl, while a member of the pursuing Bowers Gang, Patrick Hockstetter, is killed by Pennywise while searching the sewers for Ben.
Beverly Marsh, a girl ostracized over rumors of promiscuity, also joins the group; both Bill and Ben develop feelings for her. Later, the group befriends Mike Hanlon after defending him from Bowers. All the while each member of the group has encountered terrifying phenomena in various forms; these include the same menacing clown who attacked Georgie, a headless boy, a fountain of blood, a diseased and rotting man, a creepy painting come to life, Mike’s parents burning alive, and a phantom Georgie.
Now calling themselves “The Losers Club”, they realize they are all being terrorized by the same entity. They determine that “It” assumes the appearance of what they fear, awakens every 27 years to feed on the children of Derry before returning to hibernation, and moves about by using sewer lines, which all lead to a well currently under the abandoned house at 29 Neibolt Street. After an attack by Pennywise, the group ventures to the house to confront him, only to be separated and terrorized. Eddie breaks his arm, while Pennywise gloats to Bill about Georgie. As they regroup, Beverly impales Pennywise through the head, forcing the clown to retreat. After the encounter, the group begins to splinter, with only Bill and Beverly resolute in fighting It.
Weeks later, after Beverly confronts and incapacitates her sexually abusive father, she is abducted by Pennywise. The Losers Club reassembles and travels back to the Neibolt house to rescue her. Henry Bowers, who has killed his father after being compelled into madness by It, attacks the group. Mike fights back and pushes Bowers down the well to his apparent death. The Losers descend into the sewers and find It’s underground lair, which contains a mountain of decayed circus props and children’s belongings, around which the bodies of missing children float in mid-air. Beverly, now catatonic after being exposed to It’s true form, is restored to consciousness as Ben kisses her. Bill encounters Georgie, but recognizes that he is Pennywise in disguise. Pennywise attacks the group and takes Bill hostage, offering to spare the others if they let It keep Bill. The Losers reject this and reaffirm their friendship, overcoming their various fears. After a brief battle, they defeat Pennywise and it retreats, with Bill declaring that It will starve during its hibernation. Bill finally accepts his brother’s death and is comforted by his friends.
As summer ends, Beverly informs the group of a vision she had while catatonic, where she saw them fighting the creature as adults. The Losers create a blood oath by cutting each other’s hands and forming a circle, swearing to return to Derry in adulthood if It returns and destroy the creature once and for all. Stanley, Eddie, Richie, Mike, and Ben make their goodbyes as the group part ways. Beverly tells Bill she is leaving the next day to live with her aunt in Portland. As she leaves, Bill runs up to her and they kiss.
